The present technology for piezoelectric resonators requires excitation of resonant vibration mode that usually involves some form of acoustic wave coupling. Almost all the piezoelectric resonators can be categorized as Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) resonators. Two important parameters for MEMS resonators are quality factor electrical-to-mechanical coupling coefficient for RF filters. It is desirable to have high quality factor for oscillator applications and high coupling coefficient for filters.
The existing piezoelectric resonators with periodic interdigitated fingers—such as contour-mode, extensional-mode, and piezo-on-substrate resonators—excite longitudinal plate acoustic waves along one dimension. Plate-wave resonators with periodic electrodes are prone to excite other spurious modes and careful design is required in order to suppress these undesirable modes. One of the short-comings of plate-wave contour-mode resonators is insufficient piezoelectric coupling coefficient in order to build band-pass filters with large enough bandwidth for today's RF communications.